The present invention relates to calibration apparatus for calibrating ultrasonic transducer arrays. In particular, the invention relates to the calibration of transducer arrays which are intended for the inspection of curved surfaces of varying radii, such as the surfaces of the reactor and steam generator vessels in a nuclear steam generating plant during in-service inspection of the plant.
In nuclear steam generating plants, it is necessary to periodically inspect the interior of the reactor as well as the steam generating vessel for structural flaws. For this purpose, an ultrasonic transducer array having a plurality of transmitting and receiving transducers arranged at different angles are carried by a manipulating tool and moved along the surface to be inspected, a predetermined distance therefrom. In order to insure accurate readings from the ultrasonic transducer array, it is necessary that it be periodically calibrated for a particular surface which is to be inspected. This calibration cannot be effected when the transducer array is mounted on the manipulator tool, because the manipulator tool is an extremely expensive device which is in constant use. Therefore, calibration of the transducer array must be effected remotely before the array is mounted on the manipulator tool.
For this purpose, calibration test blocks are provided which have surfaces which duplicate the surfaces of the reactor which are to be inspected. The transducer array is mounted in a calibration fixture which moves the distance therefrom. Because the test surface is curved, complicated cam follower mechanisms with curved templates have previously been used for controlling the movement of the transducer array along the test block. However, this requires an inordinately large number of expensive templates, because of the variety of surfaces to be inspected and the variety of distances from the surface at which the transducer array must be maintained because of different water levels in the reactor.
Pendulum-type manipulators have been utilized in ultrasonic inspection systems for controlling the movement of the transducer array during the inspection operation. But such manipulators have not heretofore been used in a calibration fixture. Furthermore, the pendulum-type manipulators used heretofore have been relatively unstable and would not be suitable for the heavy duty use in a nuclear reactor and would not provide the desired accuracy. Furthermore, such pendulum devices have not been suitable for use with surfaces of varying radii. While pendulums with extensible arms are known, this type of adjustment would require movement of the surface being inspected with respect to the pendulum frame, or vice versa, in order to maintain the proper spacing between the transducer array and the surface being inspected. Furthermore, prior pendulum-type devices could not readily be stopped and retained at any position of the pendulum arc without the use of special locking or retaining means.